Waterproof compositions which may be used to form covering membranes for surfaces such as roofs, floors and decks have long been known to possess certain disadvantages. For example, roofing pitch, comprised of coal tar distillates having a softening point of between 50.degree. C.-60.degree. C., provides an excellent water barrier and a thick coating but requires heating far above the softening point for application. Thus an open flame, large apparatus and rapid deployment are required which, in many cases, may prohibit its use. While naphtha solvent may improve the softening point and hardening time of roofing pitch, these problems are not totally eliminated and the additional problems of flammability and fuming are presented. Moreover, roofing pitch is inappropriate for covering floors as pedestrian and vehicular traffic easily distort the membrane.
Solvent or solution resin coatings, comprising organic solvents as the evaporative vehicle, have only limited use as decking material. As the solvent evaporates during the drying of the coating, considerable irritation and odor may be caused. Also, organic solvents are not inexpensive, contribute substantially to the costs of the coating composition and, due to their high flammability, are hazardous during the manufacture and use of the coatings. In addition, coatings or covering membranes which depend on solvent evaporation for curing can generally be applied only in thin coats to allow such evaporation, and repetitive applications are required to achieve the required thickness. Water emulsion coatings such as latex compositions avoid some of the problems of the organic solvent coatings, but still require a series of thin coatings to allow water evaporation. Latex coatings are relatively expensive for roofing and decking purposes and, in the sme manner as the solvent resin coatings, are bereft of the strength and resiliency required for membranes which must bear traffic e.g. floor covering membranes.
Various elastomeric substances have been proposed and used as covering membranes, but each has heretofore been possessed of certain disadvantages. For example, catalyst cured urethanes or other polymers allow the production of impervious and resilient rubber-like material. However, such polymers are expensive and, in the same manner as the externally cured polyurethanes described below, must be carefully mixed and applied under careful temperature control, and require skilled labor and specialized static or dynamic mixing equipment which is cost intensive. When curing catalysts are employed for polyurethane elastomers, moisture must be excluded during mixing and application e.g. excessive humidity or rain will accelerate or otherwise detrimentally affect cure.
Moisture-cured polyurethane elastomers formed from polyols and isocyanate compounds i.e. externally cured polyurethanes, have been used to provide waterproof traffic-bearing surfaces for gymnasium floors, balcony surfaces, boiler room floors and rooftop parking decks to great advantage. These urethanes rely on moisture from the ambient air to effect the polymerization. Thus, the cure time is dependent upon the temperature and humidity of the air and as these parameters are somewhat unpredictable cure times of up to one week are not uncommon. If the air is too dry, the composition may totally fail to cure, and if the air is too humid, water bubbles will form. Moreover, it is often difficult to provide a coating of sufficient thickness without repetitive applications, as each coating must be thin enough to allow the migration of moisture therethrough before the top surface hardens, and each coating must be applied within the narrow parameters of temperature and humidity. In addition, because water is the curing reactant, diluents which contain even small amounts of moisture, e.g. rubber buffing dust, cannot be mixed with the prepolymer at the manufacturing facility even though such fillers would yield substantial cost savings. While fillers may be added to such compositions at the application site without causing a premature cure, particulate fillers have proven to be extremely difficult to mix with prepolymers without complex equipment due to the air and moisture entrained thereon. Additionally, air-mosture cure polyurethane elastomer decking membranes must be applied by skilled workmen, and the coating is generally applied only under the supervision of the elastomer manufacturer since the stoichiometry of these elastomers, as well as that of the catalyst cured products, must be exact.
Heretofore, water has not been added as a curing agent in polyurethane coating compositions because water produces carbon dioxide which causes foaming and deleterious bubble formation within the membrane.
The present invention provides an internally curing polyurethane elastomer formed from a polyol, an organic isocyanate, a liquid carrier containing water and a sufficient amount of an alkaline substance to absorb the carbon dioxide produced thereby. The carrier, and the resultant elastomer, also contains substantial quantities e.g. up to 60% weight rubber dust or scrap particles, thereby yielding substantial cost savings, increased resiliency and strength.
The carrier, which may be entirely comprised of water, allows the inclusion of the rubber particulate matter which is otherwise difficult or impossible to mix with polyols due to the entrained air on the particles. The water, substantially in excess of that required to polymerize organic isocyanate terminated polyol serves to form a slurry with the particulate matter to facilitate mixing with the prepolymer and to absorb a portion of the carbon dioxide produced by the polymerization reaction. Alkaline materials such as magnesium or calcium oxide are added to assist in absorbing this gas.
Further, the excess water reacts with the organic isocyanate to form a non-toxic urea complex, thus eliminating the toxic free isocyanate which is present in the polyurethane compositions heretofore known. Thus, the coating compositions or molded articles of the present invention may be used or produced in areas where prior compositions would present a threat to health and safety e.g. floor coatings for horse barns and trailers.
According to a method of the present invention, the composition is prepared from two components, each having an extended shelf life and adapted to facilitate on-site mixing, and which may be applied without the need for complex machinery or highly skilled workmen. A coating may be poured, brushed, trowelled or sprayed to vertical or horizontal surfaces to form a durable monolithic membrane having sufficient structural rigidity to withstand repeated abrasion from vehicular and pedestrian traffic. The viscosity of the composition may be easily controlled for the particular method of application, and the resiliency of the membrane may be adapted to the desired use.
As the composition cures internally, i.e., in the absence of atmospheric moisture, and is easily prepared from pumpable precursors, it may be used to form a wide variety of thick, molded articles such as mats or rubber-like structural components. It will be appreciated that the result is a low-cost elastomeric composition which may be easily mixed and applied as a thick coating by unskilled workers. Precise addition of the curing agent, heretofore required, is unnecessary since water is added in a substantial stoichiometric excess. This inclusion of water in a substantial excess of the stoichiometric amount required for polymerization allows the addition of particulate fillers without complex mixing equipment and, along with alkaline additives, absorbs the carbon dioxide produced so that foaming does not take place.